The space-time codes (STC) is a space-time two-dimensional coding technology of low complexity, and a sender adopts a manner of orthogonal coding to send signals in each antenna at the same time, thereby making full use of space and time diversities, and improving the transmitting efficiency of the system by times. But this coding solution can only be applied to a quasi-static flat fading channel, while in actual mobile communication environments, multi-path time delay is a basic characteristic of a channel model, and the fading is frequency selective, and the design of the space-time codes becomes a relatively complex problem. The orthogonal frequency division multiplexing (OFDM) technology divides a frequency-selective fading channel into multiple parallel and relevant flat fading channels, and permits the mutual overlapping of the frequency spectrums of each subchannel, therefore, the utilization efficiency of the frequency spectrum is greatly improved. As a result, it is drawing more and more attention to combine the STC technology with the OFDM-technology and fully play their advantages.
No matter the decoding of the STC or the demodulating of relevant OFDM systems, they needs to know the status information of a radio channel exactly, which then sets forth a higher requirement for the pilot allocation method of a transmitter. A good pilot allocation method assisted with a proper channel estimation algorithm can greatly improve the performance of the system, and fully play the advantages of the STC and the OFDM technologies. Whereas, an improper pilot allocation method may result in the floor effect, i.e., with the increasing of the signal to noise ratio, the bit error rate does not decrease, and this is fatal to the radio system.
In the “IEEE P802.16REVd-D5” protocol, when the subchannel allocation mode is the FUSC (Full Usage of Subchannels) and there are two transmitting antennas, the system allocates pilots according to the following rules: for an even symbol, the system allocates a variable pilot set 0 and a fixed pilot set 0 to an antenna 0, and allocate a variable pilot set 1 and a fixed pilot set 1 to an antenna 1; for an odd-symbol, the system allocates a variable pilot set 1 and a fixed pilot set 0 to the antenna 0, and allocates a variable pilot set 0 and a fixed pilot set 1 to the antenna 1. According to this allocation rule, the minimum interval between two pilot carriers is 12 carriers (without considering the fixed pilot sets). When the coherent bandwidth of an air channel is less than 12 carriers, the error of the channel estimation mainly is induced by the interpolation error, and this kind of error will not decrease with the increasing of the signal to noise ratio, thereby resulting in the appearance of the floor effect. What needs to be pointed out is that this phenomenon exists widely. For example, assume the bandwidth of the system is 20 MHz and the channel model (amended Stanford University Interim channel model-5) is the SUI-5 channel, then the coherent bandwidth of the air channel only has 7 carriers, as a result, when the user data passes the channel, there will be serious floor effect.
In the “IEEE P802.16REVd-D5” protocol, when the subchannel allocation mode is the PUSC (Partial Usage of Subchannels) and there are two transmitting antennas, the system allocates pilots according to the following rules: for an even symbol, the system allocates the zero carrier and the twelfth carrier in each cluster to the antenna 0 as pilot carriers, and allocates the first carrier and the thirteenth carrier in each cluster to the antenna 1 as pilot carriers; for an odd symbol, the system allocates no pilot carrier to the antenna 0 or the antenna 1. According to this allocation rule, the minimum interval between two pilot carriers is 12 carriers, and the floor effect described above still can not be avoided.
U.S. Pat. No. 6,298,035, entitled “Estimation of two propagation channel in OFDM”, proposes a method for increasing pilot density, wherein usable pilot carriers are allocated to two transmitting antennas at the same time, and in order to ensure that pilots of the receiving end do not intervene with each other, it is required that the pilots transmitted by the antenna 0 are orthogonal to the pilots transmitted by the antenna 1. By adopting this method, in the circumstance that pilot carriers are not increased, the pilot density of the two transmitting antennas can be rendered the same with that of a single antenna, which actually also reduces the influence of the floor effect. But this method will increase the overheads of a receiver as conducting channel estimation or synchronization, and if the method is used, it needs to make a great change to the IEEE802.16 protocol. In other words, the method is not compatible easily with the IEEE 802.16 protocol.
In general, the existing pilot allocation method does not synthetically consider decreasing the floor effect and some practical problems in application, such as the overhead of a receiver as conducting channel estimation making use of pilots and the compatibility of the protocol, and these need to be considered and improved.